1. Field of the invention
The present invention relates to a method for repairing an electrically short-circuited semiconductor device by insulating portions of the device which are short-circuited due to pin holes and the like, an apparatus suitable for practicing said method, and a process for producing a semiconductor device by utilizing said method.
2. Related background art
In recent years, various studies have been made on large area semiconductor devices such as solar cell, flat panel display, photosensor, electrophotographic photosensitive device, etc. In addition, the public attention has been focused on non-single crystal semiconductors such as amorphous silicon semiconductors to constitute those large area semiconductor devices mainly because of their reasonable production cost.
For instance, there is known a pin junction type amorphous silicon solar cell as an example of such non-single crystal semiconductor device. In this solar cell, photocarriers occur in its semiconductor layer, comprising an amorphous silicon semiconductor thin film, when light is impinged in the solar cell. The photocarriers migrate to its transparent electrode comprising a transparent conductive thin film situated on the side through which light is impinged and also to its conductive substrate situated opposite the transparent electrode by the action of an internal electric field, to thereby provide a photoelectromotive force.
The conductive thin film serving as the transparent electrode and the semiconductor thin film serving as the photosensitive semiconductor layer may be properly formed in a vacuum chamber in accordance with a plasma CVD method, a photo CVD method, a thermal CVD method, a vacuum evaporation method, or a sputtering method.
In the preparation of such amorphous silicon solar cell, due regard should be made to the problem relating to a short circuit which is often caused between the transparent electrode and the conductive substrate due to pinholes occurring at part of the semiconductor layer.
There are various causes for such pinholes to occur. For instance, in one case, since the transparent electrode is usually of some hundreds of angstrom in thickness and the semiconductor layer is usually of about 0.005 to some tens of um, minute dust particles (some micrometer to some tens micrometer in size) are deposited on the surface of the conductive substrate or they are deposited on or contaminated into the semiconductor layer during film formation to cause a removal for the conductive substrate or/and the semiconductor layer, whereby such pinholes occur at the semiconductor layer.
In other case, such pinholes are occurred when part of the semiconductor layer is removed due to its internal stress or its insufficient adhesion with the transparent electrode to furnish the conductive substrate with a region not having a desired semiconductor layer. In this case, the transparent electrode situated on the side through which light is impinged and the conductive substrate situated opposite said electrode are connected with each other through said region to be in an electrically short-circuited state. This results in significantly degrading of the characteristics required for a semiconductor device.
The occurrence of pinholes causing a short circuit is a serious problem particularly in the case of a large area semiconductor device such as solar cell, flat panel display, photosensor, electrophotographic photosensitive device, etc. In any case, it is extremely difficult to obtain a large area semiconductor device completely free of a short-circuited state region even under a clean environment substantially free of minute dust particles.
In order to solve the above problem relating to occurrence of pinholes causing a short circuit, Japanese Patent Publication 62(1987)-53958 (hereinafter referred to as Literature 1) proposes a method of making the inside of each of the pinholes occurring at the thin film semiconductor layer of a photosemiconductor to be in an electrically insulating state by perforating pinholes at its electrode layer and communicating the pinholes of the thin film semiconductor layer with the pinholes of the electrode layer. Likewise, Japanese Patent Publication No. 62(1987)-59901 (hereinafter referred to as Literature 2) proposes a method of reclaiming pinholes occurring at the thin film semiconductor layer of a semiconductor device by fusing the peripheries of the pinholes with radiation of energy beam.
FIGS. 10(A) and 10(B) are schematic views respectively for explaining the method according to Literature 1.
In FIGS. 10(A) and 10(B), numeral reference 1 stands for a translucent substrate, numeral reference 2 stands for a translucent electrode layer, numeral reference 3 stands for a semiconductor layer comprising a thin semiconductor film, numeral reference 4 stands for a back electrode layer, numeral reference 5 stands for a pinhole in a short-circuited state, numeral reference 6 stands for a pinhole provided at the back electrode layer, numeral reference 7 stands for laser beam, and numeral reference 8 stands for laser beam.
The method according to Literature 1 is to be explained with reference to FIGS. 10(A) and 10(B). That is, after a plurality of semiconductor devices have been prepared, semiconductor devices defective due to a short circuit are sorted out. As for each of those defective semiconductor devices, beam plane-scanning is performed while irradiating laser beam 7 through the other principal face of the translucent substrate 1 as shown in FIG. 10(A). When a short circuit current is measured for the semiconductor device at the time of performing the beam plane-scanning a, short circuit current does not flow when the laser beam 7 is irradiated to the portion where a pinhole 5 in a short-circuited state is present, and on the other hand, upon irradiating the laser beam 7 to the other portion where such short-circuited state is not present, a hole-electron pair is caused and migrates in the semiconductor layer 3, whereby a short circuit current flows. In view of this, the position where a pinhole 5 is present can be found for the semiconductor device by performing plane-scanning using the laser beam 7.
As for the portion of the semiconductor layer where a pin hole 5 is present, laser beam outputted from YAG pulse laser of 5.times.10.sup.6 W/cm.sup.2 in peak outputting power is radiated through the back electrode layer 4 in the way as shown by an arrow 8 to thereby remove a short-circuited state region comprising the constituent of the back electrode layer 4 which is extended to the inside of the pinhole 5. Particularly, as shown in FIG. 10(B), a pinhole 6 is made at the back electrode layer 4 formed on the semiconductor layer 3 having the pinhole 5 occurring therethrough at the time of the formation thereof such that it is coaxially in communication with the pinhole 5, whereby the inside of the pinhole 5 and that of the pinhole 6 are made to be in an electrically insulating state.
FIGS. 10(C), 10(D) and 10(E) are schematic views for explaining the method according to Literature 2.
In FIGS. 10(C), 10(D) and 10(E), numeral reference 1 stands for a translucent substrate, numeral reference 2 stands for a translucent electrode layer, numeral reference 3 stands for a semiconductor layer comprising a thin semiconductor film, numeral reference 4 stands for a back electrode layer, numeral reference 5 stands for a pinhole in a short-circuited state, numeral reference 7 stands for laser beam, and numeral reference 9 stands for a photosensor.
The method according to Literature 2 is to be explained with reference to FIGS. 10(C) through 10(E).
That is, a translucent electrode 2 is formed on a translucent substrate 1 and then, a thin film semiconductor layer 3 is formed on the translucent electrode 2. As for the device thus obtained, it is examined whether or not the semiconductor layer 3 is accompanied with a pinhole 5, by plane-scanning is performed for the semiconductor layer 3 while irradiating laser beam 7 outputted from an Ar gas laser of an extremely low outputting power through the rear side of the semiconductor layer 3 and moving a photosensor 9 arranged on the side of the translucent substrate 1 and opposite the Ar gas laser in synchronism with the scanning of the laser beam 7 in the way as shown in FIG. 10(C), to thereby examine whether or not the semiconductor layer 3 is accompanied with a pinhole 5. In this case, if such pinhole 5 is not present at the portion of the semiconductor layer 3 where the laser beam 7 is irradiated, the laser beam 7 is absorbed by the semiconductor 3 and does not reach the photosensor 9. On the other hand, if such pinhole 5 is present at the portion of the semiconductor layer 3 where the laser beam 7 is irradiated, the laser beam 7 reaches the photosensor 9, and from a signal outputted from the photosensor 9 at that time, the position where the pinhole 5 is present is detected.
When the presence of the pinhole 5 is optically detected as above described, laser beam of about 2 to 3 W/cm.sup.2 in power outputted from, for example, an Ar gas laser of 514.5 nm in oscillating wavelength instead of the laser beam 7 is irradiated to the portion where the pinhole 5 is present to thereby fuse the peripheries of the pinhole 5 with respect to the semiconductor layer 3, whereby the pinhole 5 is filled up with the constituent of the semiconductor layer 3 in such a state as shown in FIG. 10(D). The filled portion of the semiconductor layer 3 exhibits a fused state at the beginning but it is sooner or later cooled, wherein the constituent of the filled portion is changed from amorphous state to polycrystalline state or the like and the junction state is eventually broken. Thus, the filled portion finally becomes to function substantially as an insulator.
Finally, as shown in FIG. 10(E), a 2000 to 10000 thick aluminum layer serving as a back electrode 4 is laminated on the semiconductor layer 3 having the above filled portion by a vacuum evaporation technique.
The above mentioned two methods are effective in order to solve the foregoing problems relating to short circuit caused due to pinholes occurring at the semiconductor layer to a certain extent, but there still exist such problems as will be mentioned below, which are necessary to be solved.
In the case of the method according to Literature 1, there is a problem that it takes a long period of time in order to detect a number of pinholes being present at the semiconductor layer of a large area by way of the laser beam scanning process.
There is also other problem for the method according to Literature 1 that, in this method, as apparent from FIG. 10(B), the pinhole 6 is left as it is in any case, however, such pinhole must be filled up in practice. That is, in the case of a semiconductor device having such a configuration as shown in FIG. 10(B) in which the pinhole 6 is left without being filled up, water, alkaline metal, or the like are apt to enter therethrough upon use, and once water or/and alkaline metal, etc. have entered thereinto, the semiconductor device will be deteriorated shortly. However, in order to fill up such pinhole as shown in FIG. 10(B), not only a specific technique is required but also it takes time, and because of this, the resulting product becomes unavoidably costly.
Likewise, there are some problems also for the method according to Literature 2. That is, in the case of the method according to Literature 2, as apparent from what shown in FIG. 10(D), the pinhole 6 is filled up by fusing the peripheries thereof, but this process is performed prior to forming the back electrode layer 4, and because of this, a pinhole which will be caused at the time of forming the back electrode layer on the semiconductor layer 3 having the filled portion is unavoidably, left without being filled up. In addition to this, since the fill up of the pinhole 5 is performed through the laser beam-irradiating process which takes time while exposing the semiconductor layer 3 to environmental atmosphere, the quality of the semiconductor 3 is apt to deteriorate during the filling up process.
There is a still further problem for the method according to Literature 2 in that it takes a long period of time in order to detect a number of pinholes being present at the semiconductor layer of a large area by way of the laser beam scanning process and it is extremely difficult to fill up all such numerous pinholes uniformly by way of the laser beam fusing process.
In view of the above, in the case of the method according to Literature 2, if a desirable semiconductor device should be obtained, it will be costly.